This article is based on the Wikipedia entry on Topographic Map. It has been altered slightly and is presented here under the GNU Free Documentation License.

Topographic maps are a variety of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. The latter characteristic is popularly held to to define the genre, such that even small scale maps showing relief are commonly called "topographic."

Example of a topographic map with contour lines Part of the same map in a perspective shaded relief view illustrates how the contour lines of the original follow the terrain

History

Topographic maps are based on topographical surveys. Performed at large scales, these surveys are called topographical in the old sense of topography, showing a variety of landmark and landscape information. This is in contrast to older cadastral surveys, which primarily show property and governmental boundaries. Topographic surveys were originally prepared by the military to assist in planning for battle and for defensive emplacements (thus the name and history of the United Kingdom's Ordnance Survey). As such, elevation information was of vital importance.

As they evolved, topographic map series became a basic national resource in modern nations in planning infrastructure and resource exploitation. In the United States, the national map-making function migrated from the Army Corps of Engineers to the United States Geological Survey in 1878, where it has remained since.

Map Conventions

Topographic maps are also commonly called contour maps or "topo maps." In the United States, where the primary national series is organized by a strict 7.5° grid, they are often called topo quads (or quadrangles).

Topographic maps conventionally show topography, or land contours, by means of contour lines. Contour lines are curves that connect contiguous points of the same altitude (isohypse). In other words, every point on the marked line of 100 m elevation is 100 m above mean sea level.

There are several rules to note when viewing topographic maps:

• The rule of Vs: sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream. This is a consequence of erosion.
• The rule of Os: closed loops are normally uphill on the inside and downhill on the outside, and the innermost loop is the highest area. If a loop instead represents a depression, some maps note this by short lines radiating from the inside of the loop, called "hachures".
• Spacing of contours: close contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging indicates a cliff.

Of course, to determine differences in elevation between two points, the contour interval, or distance in altitude between two adjacent contour lines, must be known, and this is given at the bottom of the map. In most cases, contour intervals are consistent throughout a map. Sometimes dashed contour lines are present; these represent half the noted contour interval.

These maps usually show not only the contours, but also any significant streams or other bodies of water, forest cover, built-up areas or individual buildings (depending on scale), and other features and points of interest.

Today, topographic maps are prepared using photogrammetry interpretation of aerial photography. Older topographic maps were prepared using traditional surveying instruments.

United States Geological Survey

The United States Geological Survey (or USGS) produces several national series of topographic maps which vary in scale and extent. The largest (both in terms of scale and quantity) and best-known topographic series is the 7.5-minute, 1:24,000 scale, quadrangle. Each of these maps covers an area bounded by two lines of latitude and two lines of longitude spaced 7.5 minutes apart. Nearly 57,000 individual maps in this series cover the 48 contiguous states, Hawaii, U. S. territories, and areas of Alaska near Anchorage, Fairbanks, and Prudhoe Bay. The area covered by each map varies with the latitude of its represented location due to convergence of the meridians. At lower latitudes, near 30° north, a 7.5-minute quadrangle contains an area of about 64 square miles. At 49° north latitude, 49 square miles are contained within a quadrangle of that size.

An older series of maps, the 15-minute series, was used to map the contiguous states at a scale of 1:62,500. Each map was bounded by two parallels and two meridians spaced 15 minutes apart - the same area covered by four maps in the 7.5-minute series. The 15-minute series, at a scale of 1:63,360 (one inch representing one mile), remains the primary topographic quadrangle for the state of Alaska. Nearly 3,000 maps cover 97% of the state. The 15-minute series for the 48 states has been retired.

The next-smallest topographic series, in terms of scale, is the 1:100,000 series. These maps are bounded by two lines of longitude and two lines of latitude. However, in this series, the lines of latitude are spaced 30 minutes apart and the lines of longitude are spaced 60 minutes, which is the source of another name for these maps; the 30 x 60-minute quadrangle series. Each of these quadrangles covers the area contained within 32 maps in the 7.5-minute series. The 1:100,000 scale series is unusual in that it employs the Metric system primarily. One centimeter on the map represents one kilometer of distance on the ground. Contour intervals, spot elevations, and horizontal distances are also specified in meters.

The final regular quadrangle series produced by the USGS is the 1:250,000 scale topographic series. Each of these quadrangles in the conterminous United States measures 1 degree of latitude by 2 degrees of longitude. This series was produced by the U.S. Army Map Service in the 1950s, prior to the maps in the larger-scale series, and consists of 489 sheets, each covering an area ranging from 8,218 square miles at 30° north to 6,222 square miles at 49° north. Hawaii is mapped at this scale in quadrangles measuring 1° by 1°.

USGS topographic quadrangle maps are marked with grid lines and tics around the map collar which make it possible to identify locations on the map by several methods, including the graticule measurements of longitude and latitude, the township and section method within the Public Land Survey System, and cartesian coordinates in both the State Plane Coordinate System and the Universal Transverse Mercator coordinate system.

Other specialty maps have been produced by the USGS at a variety of scales. These include county maps, maps of special interest areas, such as the national parks, and areas of scientific interest.

A number of Internet sites have made these maps available on the web for affordable commercial and professional use. Because works of the U.S. Government are in the public domain, it is also possible to find many of these maps for free at various locations on the Internet. Georeferenced map images are available from the USGS as digital raster graphics (DRGs), in addition to digital data sets based on USGS maps (notably Digital Line Graphs (DLGs) and digital elevation models (DEMs)).

The topographic map software products offered by software-maps.com are:

• National Geographic TOPO! State series
• National Geographic Explorer 3D series
• DeLorme Topo USA 6.0.

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